1. Field of the Invention
This invention relates to a method and apparatus for shear forming a metal workpiece. More particularly, the invention relates to a punch press tool and method for fine shearing a workpiece by driving a punch or tool into and through the workpiece along a predetermined path. The directional components of the punch path and the force with which the punch is driven are such that the area of shearing is kept in compression and in a plastic state so that controlled shearing without fracture occurs.
2. Description of the Prior Art
Metal parts produced by conventional punch presses and methods often require machining or other finishing subsequent to punch forming. The need for further machining is due to the tendency of the metal workpiece to fracture ahead of the cutting tool or punch, which leaves the punched edges cleanly sheared through only a portion of the workpiece's thickness. With a typical punch shearing operation, 15% to 30% of the sheared edge has a smooth surface, followed by 70% to 85% which has a somewhat irregular fracture. Further machining to produce a clean edge requires time and equipment and increases costs of production; its elimination is therefore highly desirable.
Several prior methods are known for punch press manufacture of precision parts that do not require subsequent machining. One method is a cold forming technique called fine-edge blanking, also known as fine blanking, smooth-edge blanking or fine flow blanking. (In the case of hole rather than edge forming, it is called fine-edge piercing.). The apparatus used to execute this method uses a V-shaped impingement ring located on the pressure pad near the working edge of the punch. The impingement ring is located outside the main punch surface but close enough to the punch's cutting edge so that upon punching, the impingement ring causes the workpiece metal to flow toward the punch, thereby preventing fracture of the workpiece and eliminating all but a subsequent small burr removal step. See, "Fine-Edge Blanking and Piercing", in Production to Near Net Shape Source Book (American Society for Metals 1983).
The above process is typically used with flat or plate stock to produce pieces having a blanked edge that is perpendicular to the unworked top and bottom surfaces of the workpiece, as in conventional punch press methods. The utility and range of uses of the punch press even with the above process have thus been limited. Applications requiring a piece to have an edge or chamfer formed at an angle other than 90.degree. to the plate surfaces of the workpiece have, to applicant's knowledge, not been satisfied through the use of fine edge blanking on current punch presses.
Cold flow forming of metals using specially shaped, axially aligned dies is also known (see U.S. Pat. No. 4,177,665), as is the use of two-blow die designs for forming specially shaped parts (see U.S. Pat. No. 3,945,071). Also known is a method for fine shearing aluminum bar or other non-ferrous alloys transversely at 90.degree. by placing axial compressive stress on the bar prior to shearing. (Hungarian Pat. No. 152,686) Neither these nor other known approaches have shown themselves suitable for fine shearing metal stock to produce a broad variety of shaped surfaces on metal parts, including such commonly used items as cutting blades, saw teeth or beveled edges for welding, such that the sheared surface needs little or no finishing.
The prior work of the present inventor is also part of the background of the present invention. In U.S.S.R. Inventor's Certificate No. 407,726 a method of chamfer forming on sheet metal material is discussed. The method involves two overlapping tools exerting opposing perpendicular forces on a piece of sheet material between them. The upper tool is led straight downward into the sheet material. The lower tool is also led into the sheet material with a sideways motion that reduces the amount of overlap between the tools and forces material out. When the tools meet with minimal clearance between them the sheet material is fully cut. No description is given of the manner in which tool motion is caused; nor is there any discussion of the size or direction of forces required to produce cutting or to achieve smooth cutting without fracture. In U.S.S.R. Inventor's Certificate No. 415,068 a press tool for cutting and chamfer forming the end of a bar is discussed. The tool involves a punch with a working edge and a cutting edge opposite the working edge. A two-part die is placed below the punch, and the bar to receive the chamfer is placed on both die parts. One part of the die is movable in a direction perpendicular to the vertical stroke of the punch; the other part is stationary. As the punch descends, it acts through a cam arrangement to cause the movable part of the die to translate laterally at the same time that the workpiece is being driven downward. The cutting edge of the descending punch and the stationary part of the die vertically cut one side of the bar piece while the movable portion of the die cooperates with the working edge of the punch to cut the chamfer on the other side of the bar piece. The cut-off bar piece falls between the two die parts. No discussion of the size or direction of forces required to produce cutting or to achieve smooth cutting appears.